) This annotation must be specified for persistent map keys of type java.util.Date and java.util.Calendar ... with the ElementCollection, OneToMany, or ManyToMany annotation. Example:
@OneToMany
@MapKeyTemporal(DATE ... ) The type used in mapping java.util.Date or java.util.Calendar. (Required) The type used in mapping java.util.Date or java.util.Calendar. Since: JPA 2.0

This may be related to the fix for the problem described in this thread: Wrong data stored in date only fields. The object and its date and time fields are the same as in the referenced thread ... are using the 2015g time zone data. While we cannot see a problem with date only fields (more testing

a DateFormat instance for use with constructing Date instances. Register a DateFormat instance for use with constructing Date instances. The default is the default DateFormat instance. If the new instance

should be one of the following types: any Java primitive type; any primitive wrapper type; String; java.util.Date; java.sql.Date; java.math.BigDecimal; java.math.BigInteger. The mapped column for the primary key